The excess emission of carbon dioxide is accelerating global warming, and harmful exhaust gases and environment gases that were discharged from automobiles and industry are threatening human health. Under such circumstances, it is very important to detect the above harmful gases early and to respond appropriately to them. In particular, it is becoming important every day to measure the level of air pollution in real time and to warn when the air pollution exceeds an allowable level. In order to detect the above gases early, an ultrasensitive sensor that can rapidly and accurately monitor an extremely small amount of harmful gas at the level of sub-ppm is required.
Currently, commonly-used metal oxide semiconductor gas sensors measure the concentration of gases by detecting the change in resistance occurring when external gases are adsorbed onto the surface of metal oxides. Among metal oxides (ZnO, SnO2, WO3, TiO2, etc.), materials having a band-gap of 3.0˜4.8 eV have semiconductor properties. When external gases (NOx, CO, H2, HC, SOx, etc.) are adsorbed on the surface of such a metal oxide having semiconductor properties, the resistance of the metal oxide is changed by an oxidation-reduction process. The sensitivity of a sensor increases as the change in resistance of the metal oxide increases. Therefore, in order to improve the sensitivity of a sensor, it is preferable to increase the specific surface area of a metal oxide and to allow a sensor to have a porous structure such that gas easily moves on the surface of a metal oxide.
Electrical sensors using nanowires or nanotubes are characterized in that they can be fabricated in the form of an ultrasensitive nanosensor because the specific surface area of nanowires or nanotubes is relatively large. However, forming such nanostructures is generally accompanied by complicated processes and heat treatment must be conducted at a high temperature of 600° C. or above.
In order to miniaturize a gas sensor, it is important to form a sensor material including a gas-sensitive material into a thin layer. Such a thin layer is generally formed by screen printing, spin coating, sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD) or the like. Further, the thin layer may be formed by an electrospray process.
Screen printing is problematic in that it is difficult to form a sensor material layer to a thickness of less than 500 nm, and in that additional heat treatment is required in order to remove a binder and improve the adhesiveness between the sensor material layer and a substrate after screen printing.
Even when a sensor thin film is formed using spin coating, sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD) or the like, there are problems in that high-temperature heat treatment is required in order to obtain a nanocrystalline structure, and in that high-temperature of 450° C. is required even in-situ deposition.
More concretely, U.S. Pat. No. 7,259,109 discloses a method of forming a thin film by preparing a precursor-containing solution and then directly spraying the solution onto a semiconductor substrate. However, this method is used to form a thin film having a low dielectric constant.
Korean Patent Registration No. 10-0843191 discloses a method of manufacturing a nanofiber fiter medium containing silver nanoparticles, comprising the steps of: forming a nanofiber layer by electrospinning; and electrospraying a silver nanoparticle-containing solution onto the nanofiber layer to diffuse silver nanoparticles into the surface of the nanofiber layer. However, in this method, only silver nanoparticles are used as the metal nanoparticles.
Korean Patent Registration No. 10-0583910 discloses a method for patterning nanosized structures by electrospraying nanoparticles in order to overcome the limit of the line width of a pattern, to maintain high reproducibility and solve the problem of noise pattern occurrence. In this method, gold nanoparticles having a size of 20 nm are used as metal nanoparticles.
However, since the metal particles, such as silver (Ag) particles, gold (Au) particles and the like, mentioned in the above Patent documents, react with environmental pollutants, such as alcohols, NOx, SOx, NH3, CO2, DMMP, phenol, acetone, formaldehyde, hydrogen gas and the like, it is difficult for them to change the resistance. Therefore, they cannot be used as sensor materials.
Meanwhile, a thin film, formed by electrospraying a solution in which metal ions as metal oxide precursors are dissolved, has a surface structure which is not greatly different from that of a sensor thin layer obtained by a general sol-gel reaction or a sensor thin layer formed by sputtering or chemical vapor deposition (CVD), and has high density. Therefore, there is a problem in that it is difficult to manufacture an ultrasensitive sensor which can measure harmful gases at the ppm sensitivity level.